A pacemaker (or "pacer" as it is commonly labeled) is an implantable medical device which delivers electrical pulses to an electrode that is implanted adjacent or into the patient's heart in order to stimulate the heart so that it will beat at a desired rate. A normal human heart contains a natural pacemaker by which rhythmic electrical excitation is developed. If the body's pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, due to age or disease, an implantable pacemaker often is required to properly stimulate the heart. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Pat. No. 4,830,006.
Pacers and other implantable electrical stimulation devices generally comprise a power source and at least one lead extending from the power source to the point of stimulation. An implanted lead must be capable of conforming to the body in which it is implanted. In addition, an implanted lead is subject to repeated flexing due to heartbeat, breathing or other body movements. Pacing leads, which are attached to the myocardium, are particularly subject to rigorous and continuous flexing. Improvements in pacemaker technology have increased the life of the devices and thus increased the period for which a lead is expected to operate without failure. It has been estimated that, in a 10-year period, a pacemaker lead must withstand over 360,000,000 flexes. Therefore, the most successful leads are those that flex easily and are resistant to fatigue. Because leads must transmit electrical current from a limited power source, it is also desirable to provide leads that have low electrical resistance.
In addition, although early pacers included a single remotely-positioned electrode and therefore relied on conduction through the surrounding body to complete the circuit, newer pacers typically include a bipolar lead terminating in two, spaced-apart electrodes. This reduces the resistance of the circuit and thus reduces power consumption through I.sup.2 R losses, as most of the circuit is comprised of metal conductors, rather than through the surrounding tissue. Other of technologies have been developed that also require the placement of one or more electrodes or other devices in the heart. These include various devices that sense the state of the heart so as to optimize pacing, such as temperature sensing thermocouples and photo-oxymetry devices. Often, two or more of these devices are connected to the pacer by a single, multi-fillar lead. Typically, the number of fillars corresponds to the number of electrodes or other devices, but instances in which two or more fillars are electrically connected to a single electrode or other device are also common. Each conductor in a multi-fillar lead must be insulated from the other conductors and from the environment in which it is implanted.
Substantial effort has been employed in the development of specialized, multi-fillar leads that optimize the characteristics of flexibility and electrical conductivity while providing a surface that is suitable for implantation and that is able to resist the corrosive environment within a living body. An example of one such multi-fillar lead is described in U.S. Pat. No. 4,840,186 to Lekholm et al. It has been common to include either an electrically insulating sheathing layer, such as that disclosed in U.S. Pat. No. 4,640,983 to Comte, or to insulate individual conductors separately, as disclosed by Lekholm. Lekholm also teaches a second, additional insulating layer, in which the individually insulated conductors are embedded.
Another type of multi-fillar lead has two or more individually insulated conductors that are wound into coaxial helical coils having a uniform inner diameter. These conductors can have the same or different diameters. An example of such a lead is disclosed in U.S. Pat. No. 4,640,983. The insulation is typically made of a Teflon-type material or of a silicon-based material, or similar suitable insulating material such as are known in the art. It has been found that coiling the conductors in this manner allows the conductors to provide mutual mechanical support that further extends the life of each conductor.
Because the individual conductors in a multi-pole lead typically need to be connected to electrodes that are some distance apart, it is necessary to terminate each conductor separately. Electrical connection to an electrode or other device is made by mechanically stripping the insulation from the end of each conductor at the point where it is desired to attach an electrode. However, the act of mechanical stripping damages the wire and results in residual stresses, which shorten the life of the conductor. In addition, the step of uncoiling coiled wires from each other as required in conventional stripping procedures is time-consuming and labor-intensive and may itself result in damage to the wires. Therefore, it is desirable to provide a technique for stripping the wire non-mechanically.
Furthermore, because the most commonly used mechanical equipment uses an axial stripping process, stripping can only occur adjacent a wire end. Therefore, each conductor typically terminates at an electrode so as to avoid having a length of stripped conductor extend beyond the electrode. In order to ensure that the electrodes are spaced apart when the lead is implanted, one conductor or fillar is made shorter than the other. In these instances, where one conductor of a multi-fillar lead terminates and the remaining conductor extends beyond the first, the extending portion of the longer conductor is not mechanically supported against flexure to the degree that the conductors are mutually supported along the body of the lead. While it is theoretically possible to attach a mechanical support for the extending conductor segment so as to replace the terminated conductor in this region, each such connection or discontinuity decreases reliability of the device and increases manufacturing costs. Therefore, it is desirable to provide a technique for stripping a limited segment of one conductor at some distance from its terminal end in a manner that allows both wires to retain their individual insulation at points beyond the first electrode and extend all the way to the end of the lead, thereby providing reliable mechanical support for the longer lead in this region. While the foregoing discussion is presented in terms of a bi-fillar conductor, it will be understood that the same principles apply in multi-fillar leads having more than two conductors.
In addition, from a manufacturing standpoint, it would be most preferable to strip the wires without first having to uncoil them, so as to avoid the mechanical damage caused by the uncoiling and recoiling processes and to avoid the costly labor intensive steps associated therewith. It is desirable to strip only one wire in a given lead segment, as electrical contact between conductors would result in short circuiting. In order to avoid stripping more than one wire when multiple wires are coiled together, therefore, it is necessary to provide an apparatus and technique that can selectively strip one of several adjacent wires in a multi-wire coil.